As the screen of your phone is very small it's very easy to design a background that is too large, where there is no room to display all the fields and controls. Backgrounds larger than the phone screen are best avoided.
To give you feedback about the screen space that your background design will occupy on the phone, MoStacks on Windows attempts a close match between the window containing the card view and the card view on the phone. The default size of the card view window corresponds with the usable screen area for the card view on the phone, and fields, their labels and buttons get more or less the same relative size.
This is known as WYSIWYG: What you see is what you get.
As there are considerable differences between a Windows program using Windows fonts and a Symbian program, actually it's only "What you see is more or less what you get". Especially if you push it to the limit you will have cases where it looks on Windows as if there was just enough room for all your fields and controls whereas the final result on the phone, the one that really counts after all, is too large.
If you look closely you will notice that in the card view window MoStacks will draw text of a given point size somewhat larger than Windows (and even MoStacks itself in its other windows) normally does. This is not a bug but deliberate: At "true" point sizes the fonts are too small to achieve a good correspondence between Windows and phone.